316L and DI Water Rusting

[adixs71]

Hello,
We have a 316L Tank with 140 deg DI water. We are heating the water with a strip heater bonded on the non-wetted side of the tank.
In the areas where the heaters are bonded, we are seeing some rust on the wetted side. The heaters are bonded with a heat sink compound.
The DI water does have localized boiling but this is common in our tanks.

Any thoughts of why this may be happening? Poor grade of 316L?

 

[EdStainless]

1. The tank may not have been passivated properly when it was built.
2. is this tank open to air? You may have excessive levels of CO2 driving down the pH and making it more aggressive.

Turn the heater down, boiling does not improve your heat transfer much.

I would suggest cleaning the rusted areas with pickling paste and then nitric acid passivating the entire tank.

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[CoryPad]

I think Ed's #1 is the likely cause.

 

[adixs71]

Thanks Ed, yes it is open to air.

One other item I left out was they are electropolished. This has happened on some recent non-electropolished tanks as well. So I am inclined to think that the EP is a non-issue.

Why does the rust only appear around the heater regions? Does this speed up the oxidation process?

Maybe this rust is not really rust and "burn" from the heaters. Looks like rust as it is more brownish than blue heat discoloration.

 

[EdStainless]

Just because they are EP does not mean that they were properly cleaned. Does your spec prohibit the buffing of the tank? Do you require a nitric acid passivation after EP?
You should on both counts.
Turn the power down. Once water starts to boil you probably are loosing heat transfer capacity. The surface can get very hot since the steam formed does not transfer much heat.
While passivation is the most likely issue I still question the water quality. When you boil you will get localized concentration of impurities.
What is your conductivity (total)? pH?

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